1. Field of the Invention
The invention relates to a process for converting hydrocarbons. The process of the invention associates a distillation zone with a reaction zone for hydrocarbon conversion which is at least partially external to the distillation zone into which an effluent comprising hydrogen is introduced. Thus this process can selectively convert hydrocarbons separated from a hydrocarbon feed by means of the distillation zone, in a reaction zone associated with withdrawing the feed for the reaction zone from the distillation zone and re-introducing the converted feed into the distillation zone.
More particularly, the process of the invention is applicable to selective reduction of the quantity of light unsaturated compounds (i.e., containing at most six carbon atoms per molecule) comprising benzene and possibly olefins in a hydrocarbon cut essentially comprising at least 5 carbon atoms per molecule, with no substantial loss of octane number.
2. Description of the Prior Art
The general trend now is to reduce the quantity of benzenes and olefins (unsaturated compounds) in gasolines, because of their known toxicity.
Benzene has carcinogenic properties and thus the possibility of it polluting the air must be limited as far as possible, in particular by practically excluding it from automobile fuels. In the United States, reformulated fuels must not contain more than 1% by volume of benzene; in Europe, it has been recommended that a gradual decrease towards that value be made.
Olefins are known to be among the most reactive hydrocarbons in photochemical reactions with oxides of nitrogen, which occur in the atmosphere and which lead to the formation of ozone. A rise in the concentration of ozone in the air may be a source of respiratory problems. It is thus desirable to reduce the amount of olefins in gasolines, and more particularly of the lightest olefins which have the greatest tendency to vaporize when handling a fuel.
The benzene content of a gasoline is very largely dependent on that of the reformate component in that gasoline. The reformate results from catalytic treatment of naphtha intended to produce aromatic hydrocarbons, principally comprising 6 to 9 carbon atoms per molecule and the octane number of which is very high endowing the gasoline with antiknock properties.
Because of the toxicity described above, the amount of benzene in the reformate must be reduced by a maximum.
The benzene in a reformate can be hydrogenated to cyclohexane. Since it is impossible to selectively hydrogenate benzene in a mixture of hydrocarbons also containing toluene and xylenes, that mixture must first be fractionated to isolate a cut containing only benzene, which can then be hydrogenated.
International patent application WO 95/15934 describes a reactive distillation which aims to selectively hydrogenate diolefins and C2-C5 acetylenic compounds. The catalytic hydrogenation zone is completely internal to the distillation column, which means that the hydrogen cannot dissolve properly in the feed and the pressure cannot be increased.
A number of processes have been described in which the catalytic benzene hydrogenation zone is internal to the distillation column which separates benzene from other aromatic compounds, which cuts the cost of the apparatus. Such a process has been described in U.S. Pat. Nos. 4,232,177, 4,307,254, 4,336,407, 3,629,478, 4,471,154 and 3,629,478. It appears that the pressure drop across the catalytic bed(s) in that process means that an intimate mixture between the liquid phase and the gas stream containing the hydrogen cannot be obtained. In that type of technology where the reaction and distillation proceed simultaneously in the same physical space, the liquid phase descends through every catalytic bed in the reaction zone in a trickle flow, and thus in threads of liquid. The gaseous fraction containing the fraction of vaporized feed and the gas stream containing hydrogen rise through the catalytic bed in columns of gas. In that arrangement, the entropy of the system is high and the pressure drop across the catalytic bed(s) is low. As a result, operating using that type of technique does not enable hydrogen to dissolve readily in the liquid phase comprising the unsaturated compound(s).
A number of processes have been described in which the reaction zone is external to the distillation column with withdrawal of the feed to be converted from one level in the column and re-introduction of the converted effluent into the column. Such a process has been described in U.S. Pat. No. 4,503,265 and in International applications WO 93/19031, WO 93/19032 and WO 94/13599 for application to alkylether synthesis. Similarly, U.S. Pat. No. 5, 177, 283 describes this technique for aromatic hydrocarbon alkylation.
The Applicant's European patent application EP-A-0 781 830 describes a process for hydrogenating benzene using a distillation column associated with a reaction zone which is at least partially external. The effluent is recovered overhead from the column, then arrives in a drum via a condenser from which drum the desired liquid product is recovered and it is observed that certain losses of hydrocarbon compounds can occur, in particular a loss of compounds containing 5 carbon atoms.